This invention relates in general to sewing machines and in particular to a new and useful method and apparatus for determining and adjusting the amount of advance of a plurality of plies of workpieces for the purpose of coordinating them.
For the performance of various switching and control processes in the automation of certain sewing jobs, as for instance in the sewing together of two work plies with exact end edges of precise length, it is advantageous to determine exactly the actual amount of advance of the work per forward step of the feeder or a possible relative motion between two plies.
Through German Pat. No. 23 61 375 a sewing machine for the sewing together of two plies in proper position is known where for each ply a measuring wheel scanning their amount of advance is provided, which wheel delivers pulses for a control circuit acting on the feeders. It has been found that between the plies and the measuring wheels a slip may occur which falsifies the measurement result and which depends, inter alia, on the cloth type of the workpiece, the direction of the warp and weft threads and the rate of feed. For this reason this measuring method is imprecise, in particular for relatively long seams.
Through German Pat. No. 32 16 993, a method for measuring the amount of advance of a workpiece in the end section of a seam is known, where the work follow-up edge, extending at an angle to the edge being worked, is scanned by a sensor device which comprises essentially two sensors arranged spaced one behind the other. The sum of the pulses, formed in the interval between the response of the two sensors, of a pulse generator connected with the main shaft of the sewing machine; is compared with a number of pulses which is calculated by a microprocessor as a function of the distance between the scanning points of the two sensors and as a function of the set stitch length of the feeder of the sewing machine. From the difference of the pulse sums, if one exists, the size of the slip between feeder and workpiece is determined.
As the beginning and end of the pulse counting process are triggered by the scanning of a workpiece edge, the workedges must be well trimmed and must not be fringed if great precision is to be achieved. Since here always the followup edge of the workpiece extending at an angle to the edge being worked is being scanned, this measuring method can be carried out only in the end section of a seam, for example for exact approach of the seam corner.
Lastly through German Pat. No. 1,302,988 a control is known where various switching means of a sewing machine are controlled by the scanning of reflecting marking points applied on a template lying on the workpiece itself, Here it is necessary, if a high work related switching accuracy is to be obtained, that the marking points are exactly correlated with those points of each workpiece at which certain switching processes are to be triggered. This prerequisite is indeed fulfilled with the use of templates bearing marking points, provided the template and the workpiece are always exactly aligned; but the expense of producing such templates is warranted only for a large number of identical workpieces. Even if the marking points are applied directly on each single workpiece, to make sure of the stipulated accuracy of placement a template will again be used, so that this type of marking is meaningful only for a large number of identical workpieces also.